1. Field of the Invention
The present invention relates to a method of adjusting the optical axis of a light transmission path that includes various optical components. More particularly, this invention relates to an optical axis adjustment method for adjusting the axis of an optical path that includes optical components such as optical fibers, optical fiber arrays, lenses, light-emitting elements, light-receiving elements, semiconductor lasers, optical waveguides, mirrors, and so forth, and to a storage medium recorded with a processing program that executes said adjustment method.
2. Description of the Prior Art
In general, an optical path of an apparatus or system used in optical communications, optical measurements, laser processing and the like has many optical components connected thereto, such as optical fibers, flat plate waveguides, semiconductor lasers, and mirrors. As such, high-speed, high-precision adjustment of optical paths between optical components has become an important issue.
FIG. 1 shows how optical transmission loss can occur between components. In the case of FIG. 1(a), loss is caused by axial misalignment between two optical fibers; by a gap between fibers in FIG. 1(b); by the intersection between the axes of fibers shown by FIG. 1(c); and in the case of FIG. 1(d), by the end of one fiber being imperfectly shaped. Such problems are resolved mainly by using a connector that lines up the axes mechanically. However, associating other optical components, such as connecting an optical fiber and a flat plate waveguide, a semiconductor laser and a lens, and the lens to an optical fiber, for example, is difficult owing to the constraints imposed by the processing. Therefore, axial alignment of optical components is effected by using a precision positioning apparatus such as a stage that can be moved in fine increments.
In the example illustrated by FIG. 2, which relates to the adjustment of the axes of an optical fiber 10 and a light-receiving element 11, the optical fiber axis has five degrees of freedom: two displacements (x, y) perpendicular to the optical axis, two rotational amounts (xcex8x, xcex8y) about the perpendicular axes, and one displacement (z) along the optical axis. Taking the degrees of freedom as variables, the maximum intensity of light that has passed through the optical path can be found in accordance with a given search algorithm. Below, the above variable is referred to as an axial coordinate value. In the prior art, the hill-climbing method is used as the search algorithm. The hill-climbing method applied for adjustment of FIG. 2 will be explained with reference to FIG. 3.
First, with respect to the light-receiving element, the optical fiber is moved axially at a predetermined feed pitch to find a position at which the peak intensity can be obtained, from comparison of the respective intensities of the light received by the light-receiving element at the positions to which the optical fiber has been moved. This procedure is repeated for each of the X, Y and Z axes insofar as the intensity of the light received increases. Also, as disclosed in JP-A-HEI 09-311250, in order to reduce the search time, the feed pitch can be set using a plurality of steps, with coarse adjustment being followed by fine adjustment.
FIG. 4 shows the case of a local intensity peak rather than a monotonic increase distribution. Because the search will be terminated at such a local peak, in some cases, with the prior-art hill-climbing method, the search will not reach the true peak. In particular, when the optical path to be adjusted has a large number of degrees of freedom, such as when there is a plurality of components such as semiconductor laser, lenses, and optical fibers laid out in series along a single optical path that are to be connected together, there are many local peaks, which can cause the search to terminate before a sufficient received-light intensity is obtained.
In addition to the hill-climbing method, there is a method using vector searches, as described in JP-A-SHO 62-75508. This applies gradient measurement to effect a vector search. Although this requires fewer transitions than the hill-climbing method, it lacks means to confirm a true peak and therefore can be stopped at local peaks. There is also the method disclosed by JP-A-HEI 06-226415 and JP-A-HEI 07-62823 in which the search time is reduced by assuming a set shape for the received-light intensity distribution and using the results of the measurements to infer the shape parameters. However, this method also suffers from the problem that the search may be terminated at local peaks. Moreover, when the target intensity distribution differs markedly from the assumed distribution, it becomes impossible to perform an effective search without adding or modifying algorithms.
Thus, as described above, the prior art technology of adjusting the optical axes of optical components has problems that include much time required for the search, and trapping at local peaks that prevents the full received-light intensity being ascertained. Particularly with respect to the manufacture of optical modules in which optical elements such as light receivers and emitters are connected to optical fibers, the lengthy time required for axial adjustment increases the number of manufacturing steps, thereby decreasing productivity and raising costs.
Moreover, while the above explanation concerns axial adjustment between optical fibers and other optical components, the same problems arise when the optical path includes portions where the light is propagated through air. For example, in the case of a light transmission path in which a laser is used to transmit control signals or video signals between a movable section and a fixed control section of an apparatus, a laser beam is transmitted from an emission unit to a receiving unit. In such a case, it is necessary to effect axial adjustment of five degrees of freedom at the emission unit, as in FIG. 2. This axial adjustment requires much time and much labor. Also, when an optical path has a plurality of mirrors combined to reflect the alight, the same problems arise with respect to adjusting the positions of the mirrors. In laser processing, for example, in which the laser beam is transmitted precisely to a target point on the workpiece, the mirror angle is adjusted by hand while visually checking the mirror position. However, in a radiation or high-temperature environment or other such environment that does not allow a person to come close, adjustment has to be performed automatically from a remote location, using a sensor such as a CCD camera. In such cases, adjustment of multiple degrees of freedom of multiple mirrors is required. When this involves adjusting the position of mirrors several tens of centimeters in diameter, a major problem is that as a result of gravity-induced flexing or the like, displacement of one axis of a mirror affects the displacement of other axes. For example, displacing the X axis of a movable mirror can result in the simultaneous displacement of another axis, for example the Y axis, which should not be displaced. For this reason, the relationship between the amount of axial displacement and the deviation from the target position does not become monotonic. Moreover, when the hill-climbing method is used to effect automatic adjustment and adjustment is thrown off by the presence of local peaks, the amount of displacement from the target position becomes very large. In addition, in most cases in which axial adjustment is required in such an environment, evaluation values for adjustment include noise caused by axial deviation arising from extraneous mechanical noise and vibration, air fluctuations and so forth, causing divergence of searches based on the gradient method. That is, measured gradient values differ entirely owing to the presence of noise, and with more repetitions, searches go off in an increasingly unrelated direction.
Therefore, in consideration of the aforementioned points, an object of the present invention is to provide an optical axis adjustment method for a light transmission path that enables local peaks to be avoided and adjustment of optical axes of multiple degrees of freedom to be effected at high speed and in parallel, does not require addition or modification of algorithms to handle a shape of a transmitted light intensity distribution, and also has strong resistance to disturbance.
To attain the above object, the present invention provides a method for adjusting an optical axis of a light transmission path that includes a plurality of optical components, the method comprising using an adjustment apparatus to sequentially change an optical axis of a designated single optical component or multiple optical components among the plurality of optical components in accordance with a probabilistic search technique to obtain an optimum evaluation value for light transmitted through the light transmission path.
Since in accordance with the adjustment method of this invention the optical axis of a light transmission path that includes a plurality of optical components is adjusted by sequentially changing the optical axis of a specific optical component or multiple optical components in accordance with a probabilistic search technique to optimize the evaluation values of light transmitted through the light transmission path, it is possible to avoid local optimal evaluation values and effect high-speed axial adjustment of multiple degrees of freedom in parallel. Moreover, there is no need to add or modify algorithms to handle the shape of the transmitted light intensity distribution, and search performance is less subject to degradation caused by disturbance.
Here, the evaluation value of light transmitted through a light path can be represented by a function F which takes as arguments the coordinate values of the plurality of adjustable optical axes that the light path includes. Optimizing the evaluation value of the transmitted light is equivalent to finding the optimum solution to the function F. The present inventors took note of this point and discovered that genetic algorithms and other probabilistic search techniques are applicable to the axial adjustment of an optical transmission path.
Genetic algorithms are a type of probabilistic search technique, that (1) are effective in wide-area searches, (2) do not require differential values or other derived information other than the evaluation function F, (3) are readily implemented, and (4) are not readily influenced by disturbance. Therefore, in the present invention, a genetic algorithm can be used in searches for optimum optical axis coordinate values with the adjustment apparatus. Also, after sequentially changing the optical axis according to a genetic algorithm, the hill-climbing method can be used to find the optimum axis coordinate values, thereby making it possible to shorten the adjustment time.
In addition, in the event that the aforementioned evaluation function F satisfies special conditions, it is possible to improve the search efficiency by replacing the genetic algorithm with the simulated annealing method, which is also a probabilistic search technique. Therefore, in the present invention, the simulated annealing method may also be used in searches for optimum optical axis coordinate values with the adjustment apparatus. Although the performance obtained thereby is lower than that obtained with a genetic algorithm, the adjustment time can be shortened.
Moreover, the coordinate values can be measured while the optical axis is being changed by the adjustment apparatus, stored in a memory paired with the evaluation value of the transmitted light, and the pair of axial coordinate values with the largest evaluation value among the pairs taken as a local optimum solution. Doing this makes it possible to improve search efficiency and greatly reduce the search time. Also, when the adjustment apparatus is searching for the optimum coordinate values, light intensity may be used as the transmitted light evaluation value. Again, when the adjustment apparatus is searching for the optimum coordinate values, the amount of positional deviation of the light may be used as the transmitted light evaluation value.
With respect to the above-described optical axis adjustment method of this invention, the light transmission path may include optical fibers and optical fiber arrays. The light transmission path may also include optical components such as lenses, light-emitting elements, optical waveguides, mirrors, and so forth.
The adjustment apparatus used in the optical axis adjustment method may include an electronic computer and a storage medium that can be read by the electronic computer. When using a probabilistic search technique to search for optical axis coordinate values for one or a plurality of optical components that provide the optimum evaluation values with respect to light transmitted through the light transmission path, this will make it possible to readily and securely process the searches in a short space of time.
The present invention also includes a storage medium recorded with an adjustment program that is executed by the electronic computer to use probabilistic search techniques to search for optical axis coordinate values for one or a plurality of optical components that provide the optimum evaluation values with respect to light transmitted by a light transmission path. By enabling the adjustment program executed by the computer for the optical axis adjustment method of this invention to be stored, the storage medium enables adjustment of optical apparatuses to be carried out anywhere.
The above and other objects, further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.